A prior patent document of Japanese Patent Application laid open No. JP2008-212795A has disclosed an optical semiconductor module. The optical semiconductor module disclosed therein includes a semiconductor laser diode that emits laser beam, a concentrating lens that concentrates the laser beam emitted from the semiconductor laser diode, and an optical connector that outputs the laser beam concentrated by the concentrating lens in a transmission line secured by a fiber ferrule. The optical connector includes the fiber ferrule that provides an optical fiber having an incident surface through which the laser beam enters and an optical attenuating portion that covers the incident surface. The optical attenuating portion may be a polarization glass. The optical attenuating portion has transmittance for the laser beam depending on a rotational position thereof. Also, the semiconductor laser diode, the concentrating lens, and the optical connector are optically aligned such that the laser beam projected on the incident surface in a spot size thereof shows a diameter smaller than a diameter of a core of the optical fiber.
In a transmitter optical module, even when the semiconductor laser diode (hereinafter denoted as LD), the concentrating lens, and the optical fiber are adequately aligned to each other, an optical power of the LD operating under a preset bias current, namely, the optical power externally output from the module and coupled to a lens, sometimes exceeds a preset power. Reducing the bias current to set the optical power in the preset power, the LD in a resonance frequency thereof also lowers to degrade high frequency performance thereof.
Accordingly, as disclosed in the prior patent document, sliding the optical connector along the Z-axis, along an optical axis, which offsets the incident surface of the optical fiber from a focal point of the concentrating lens, a portion of the laser beam entering the core of the optical fiber relatively decrease, which is often called as a defocus.
A transmitter optical module that implements a plurality of LDs multiplexes optical signals output from the LDs onto an end surface of a unique optical fiber. The defocus described above may optionally adjust the optical power coupled with the unique optical fiber only for one of the LDs. However, for instance, when the transmitter optical module is necessary to adjust the optical power for four LDs, a subject is left that scattering in the optical coupling efficiency of components for multiplexing the optical output power of the respective LDs or for wavelength-multiplexing the laser beam of the respective LDs increases.
Because four laser beams are coupled on the end surface of the optical fiber through the unique concentrating lens, when the defocus adjusts the optical power for only one of LDs, the laser beams of the rest of the LDs are not always adequately adjusted. That is, optical axes of the LDs each directing the concentrating lens ideally align with the optical axis of the concentrating lens; but they are practically misaligned in respective LDs. Accordingly, even when the end surface of the optical fiber is shifted along the optical axis of the concentrating lens, the laser beams of the LDs do not always enter the end surface along the optical axis of the concentrating lens. Also, optical defocus amounts are often different from respective LDs, the LDs are not always defocused in respective optimum amount.
The present invention, based on the subjects above described, is to provide an transmitter optical module and a multi-lane transmitter optical module showing stable optical output power.